1. Field of the Invention
The present invention relates to a composite antenna, and in particular, to a composite antenna that can be used for radio frequency identification (RFID) systems.
2. Description of the Related Art
RFID systems, for use as article management systems, have been developed in which non-contact communication and information transmission are performed between a reader/writer generating an induction field and an IC chip (also called an IC tag or a wireless tag) that stores predetermined information and is attached to an article or a container. The IC chip can communicate with a reader/writer by being connected to an antenna.
As a tag antenna for mounting an IC chip, the tag antenna disclosed in Japanese Unexamined Patent Application Publication No. 2006-295879 is known. This tag antenna includes an inductor and a dipole antenna. The dipole antenna has wide portions at both ends thereof, and the length of the dipole antenna is shorter than a half wavelength using the wavelength reduction effect of the wide portions. The inductor is connected to respective dipole antenna portions provided on both sides of a feeder. This inductor is used to adjust the impedance between the antenna and the IC tag arranged at the feeder.
Usually, a wireless tag exchanges information in relatively long distance communication. However, it is preferable in some cases to be able to perform short distance communication for particular information. When a plurality of wireless tags are arranged in proximity to a reader/writer, it is preferable that communication with the plurality of different wireless tags be established at the same time. However, the above-described tag antenna causes a problem in short distance communication, since a dipole antenna works better as a long distance electric field antenna. In addition, a dipole antenna has a problem in which a point at which an electric field becomes zero, i.e., a “null point” is generated in accordance with operating conditions such as a communication distance, leading to unstable communication.